Umm, perhaps I don't understand how you're calculating the damping factor. With the nCore having an output impedance of about 1.3 mOhms, there's no way you're anywhere near a DF of 1. If you assume 2.6 ohms for the paralleled woofers, you'd still get about 2.6/.0013=2,000 damping factor. Am I doing this wrong?

EDIT: Even if you divide in half again, you still have a damping factor of 1000. Perhaps you misread the chart and were thinking the nCore has an output resistance of 1 Ohm?

Umm, perhaps I don't understand how you're calculating the damping factor. With the nCore having an output impedance of about 1.3 mOhms, there's no way you're anywhere near a DF of 1. If you assume 2.6 ohms for the paralleled woofers, you'd still get about 2.6/.0013=2,000 damping factor. Am I doing this wrong?

EDIT: Even if you divide in half again, you still have a damping factor of 1000. Perhaps you misread the chart and were thinking the nCore has an output resistance of 1 Ohm?

With two woofers in series, one of the woofers act as a series impedance for the other woofer. Thus, the damping factor can never be higher than 1.

I take it StigErik considers one woofer's impedance as a series impedance added to the amp's output impedance when calculating the DF the other woofer sees. If you take DF literally you could reason like that. After all, when you knock on one woofer its motion will hardly be damped. The other woofer will even move in the opposite direction. That's what I meant earlier by saying that antiphase motion isn't controlled. Imagine however knocking on both woofers at the same time in the same way. At that moment you will feel significant resistance.
If the woofers are identical and they aren't knocked on externally their motion resulting from the audio signal will of course be damped pretty much in the same way as if they'd had their own amp. So no, series connection does not cause the woofers to become undamped as far as the movement is generated electrically.

Now for the usual disclaimer: a DF of 1000 does not imply 100-fold better damping than a DF of 10. This is quite a good thing since loudspeaker design relies on some resonance with a defined Q i.e. one with a known and finite amount of damping. What makes it finite is the voice coil resistance (plus some known resistance in the crossover). This resistance is electrically connected in series with the amp and is in the several ohms region. If this amp has 100 milliohms or 1 milliohm of output resistance the change in Q of the loudspeaker system is absolutely minimal. It would be rather different if the voice coil were superconducting. In that case a 1 milliohm amp would indeed have a hundredfold greater stopping power than a 100 milliohm amp. You would feel the difference in that in the former case the woofer would not budge when pushed. Has anyone ever known a speaker of which the woofer could not be moved by hand when the inputs are shorted? Thought not. Voice coils aren't superconducting. It amazes me when people report that a certain amplifier (usually one of mine) is wonderfully capable of stopping the woofer motion and more, that they think they can actually see it happening. This is a physical impossibility.

Low output impedance has some value, luckily. Speakers are nonlinear things, so a speaker's back EMF is distorted. If you use biwiring on a very low-impedance amp you can stop distortion generated in, say the woofer, from electrically cross-talking into the tweeter. With moderate impedance amps you can only get the same result with biamping. I think this is the only sensible explanation why biwiring sometimes sounds different. I'm not saying the effect is very big but it's hard to argue that it is always inaudible. That's good news for me because Ncore has unusually low output impedance and if it didn't have any value I'd be embarrassed about boasting about it

The current capability of the NC400 is around 27A. The UcD400's is 17A. So you can't safely halve the load impedance and be certain that the amp won't ever run out of steam. In this respect I'd like to second FB: just get two UcD400ST's to drive the woofers instead. I think it unlikely that you'd hear a difference between Ncore and UcD in a pure woofer/subwoofer application. Likewise the HG (and certainly HxR) option is overkill for woofer work. One of our bigger industrial customers insists they do hear a clear difference between UcD and a commonly used Danish amp (to the advantage of UcD of course) though. They're serious people so I'll take their word for it. So we probably can't say the quality of the woofer amp doesn't matter at all, but it's perfectly sensible to be rational about the LF amp and splash out on the mid/high amps only.

I'm in a situation where I will need more power amps anyway, since I will double the number of woofers. I have four UcD400's now. The options are to get four more UcD400's, or get four NC400's and load them with half the impedance.

If anyone would like to explain why woofers wired in series sounds sloppy compared to parallell wiring... even if they technically/theoretically should not.... ?

StigErik, on what do you base your statement about serial connected woofers to sound sloppy?

I think an important part of Bruno's answer was "If the woofers are identical..." Identical means also indentical enclosure and acoustical loading. Both also have an effect on the impedance seen by the amp. And we all know from measuring T&S parameters that woofers are never fully indentical. If the above makes a difference, I do not know.

Anaother important fact is power I think. If you hear (feel) only the difference when playing pretty loud, then this will probably be your explenation.

Example: woofers are perfecty 8 ohm (flat impedance) and you amp delivers 400 Watt into 8 ohms. If it is a perfect amp, then it will deliver 800 Watt into 4 ohms. But also only 200 Watt into 16 ohms (limited by output voltage).

This means the difference of available power is quite big between the 2 configurations, and when playiing loud , you just might need this extra headroom...

I have four UcD400's now. The options are to get four more UcD400's, or get four NC400's and load them with half the impedance.

Well it's not good for my business to tell you this but since we're talking about the woofers I think this one is a no-brainer. Get extra UcD400's to complement the ones you already have.

Quote:

Originally Posted by StigErik

If anyone would like to explain why woofers wired in series sounds sloppy compared to parallell wiring... even if they technically/theoretically should not.... ?

It's unlikely that the woofers are identical because at the very least the acoustic loads are different. I replied to a similar question earlier:

Quote:

Originally Posted by Bruno Putzeys

Loudspeaker designers tend to avoid series wiring because any antiphase motion of the woofers is not damped at all. I don't know how serious the problem is with modern units but subharmonic oscillations have been reported in the past.